1. Technical Field
This invention relates to medical diagnostic and therapeutic apparatus and methods, and more particularly to an implantable cardioverter defibrillator method and apparatus for delivering a preemptory cardioversion or defibrillation pulse to a patient upon detection of predetermined physiological information related to cardiac function. The method and apparatus of this invention provide a means of delivering cardioverter defibrillator therapy to a patient having an implantable cardioverter defibrillator as soon as possible after the onset of a cardiac event such as arrhythmia (for example ventricular tachycardia or ventricular fibrillation) increase the chances of patient survival from such an event.
2. Background Information
The implantable cardioverter defibrillator (ICD) is a well recognized and important tool for managing the health of patients who have a history of heart problems, for example in the control of arrhythmias. Cardioversion involves the administration of a low energy shock (typically 0.1-5 joules (J)), via implanted electrodes, converts the arrhythmic heart rate. The ICD is further capable of resuscitating patients who experience cardiac arrest, via defibrillation. Defibrillation of the heart is accomplished by applying a high energy electrical waveform to cause the cessation of rapid uncoordinated contractions of the heart (fibrillation) and restoring the normal beating of the heart.
Referring to FIG. 1, currently, the process of converting a suspected arrhythmia or fibrillation to a normal heart rhythm, via an implantable cardioverter defibrillator (ICD), involves the following steps:
1. Detection-- Detecting, during a time period 12, either a fibrillation 10 or another high rate arrhythmia. Detection may be accomplished by detecting a heart rate above a predetermined threshold value for a specified number or count of heart beats or by another or an additional algorithm that looks at waveform morphology, rate acceleration, rate stability or other criteria.
2. Charging-- If cardioversion or defibrillation (vis-a-vis antitachy pacing) is warranted, charge storage means, typically capacitors, are charged during time period 13. The charging process can take from one to fifteen seconds depending upon the selected energy level and the state of the batteries.
3. Confirmation-- After charging the capacitors, non-committed type devices reexamine ("take a second look at") the heart rate and discharge the capacitor internally if the heart rate has deceased spontaneously. If the rate still exceeds the threshold for a specified number of intervals, the charge is held. In committed devices, this step is omitted.
4. Shock-- The energy stored in the capacitor is delivered 14 to the heart via implanted electrodes.
The above-referenced process, from detection of arrhythmia (including fibrillation) to capacitor charge up, takes between 5 to 25 seconds to complete. In the meantime, the patient may be fibrillating for 10 to 25 seconds before a defibrillation shock is applied. However, it is well established in the literature that there is a direct relationship between the elapsed time from onset of fibrillation to defibrillation and the minimum energy required therefor, the defibrillation threshold. Thus, it is in the best interest of the patient to receive therapy as soon as possible after onset of the arrhythmia, especially if the arrhythmia is fibrillation.
The typical ICD utilizes a rather large capacitor or assemblage of capacitors to store charge and to discharge a shock or pulse to the patient. It is known that a given energy delivered in the form of high currents (or voltages) for shorter durations are more effective than lower currents delivered for longer duration's. It is further known that a small value capacitor takes less time to charge to a given voltage than a large value capacitor and that a small value capacitor can deliver a given amount of energy in a shorter period time than a large value capacitor.
In summary, various implantable cardioverter/defibrillator devices and/or methods have been used or proposed in the past to defibrillate or otherwise control the activity of the human heart. However, these implantable cardioverter/defibrillator devices and methods have significant limitations and shortcomings, foremost of which is the time delay from detection of a cardiac event to capacitor charge-up and discharge.
Despite the need in the art for an implantable cardioverter/defibrillator apparatus and method which overcome the shortcomings and limitations of the prior art, none insofar as is known has been developed or proposed. Accordingly, it is an object of the present invention to provide an implantable cardioverter/defibrillator method and device which is capable of delivering a shock in the shortest time possible, to thereby defibrillate successfully more often and to utilize less energy in the process. A further object of this invention is to provide an apparatus and method which utilizes a relatively small charge storage mechanism which is able to deliver a preemptory shock at an early stage in a cardiac event. Another object of this invention is to provide an apparatus and method which anticipates the onset of fibrillation or other arrhythmia and charges a preemptory shock capacitor before the fibrillation or other arrhythmia is actually detected to thereby deliver a cardioverting shock immediately upon detection or confirmation of the presence of such fibrillation or other arrhythmia. It is a further object of this invention to provide a method and apparatus which is reliable, safe, compact (in the case of the apparatus) and which otherwise overcome the limitations and shortcomings of the prior art.